Although natural gas at the well-head is often more than 95% methane, for pipeline transport and ultimate use, impurities such as hydrogen sulfide, carbon dioxide, water and lower aliphatic hydrocarbons must first be removed. Hydrogen sulfide is presently removed by contacting the crude gas with a solution of an aliphatic amine which complexes with this contaminant and allows its removal. However, this process is complicated by the need to continuously remove the complexed material, decompose the complex and recycle the amine.
An alternative approach to hydrogen sulfide removal, exemplified by U.S. Pat. No. 4,493,716, the entirety of which is incorporated herein by reference, involves selective diffusion through a polysulfide membrane. Raw natural gas is purified by allowing hydrogen sulfide and carbon dioxide in the gas to rapidly and selectively diffuse through a thin film of polysulfide resin. The polysulfide resin film is supported on a microporous film of stretched polypropylene or expanded polytetrafluoroethylene (PTFE).
Despite such advances, practical membrane-based systems for removing hydrogen sulfide from gas streams has remained an elusive goal. A review of membrane processes (R. W. Baker, Ind. Eng. Chem. Res. 2002, 41, 1393-1411) makes no mention of commercial membrane processes to remove hydrogen sulfide from natural gas or refinery gas streams.